Rectifying resistance switching behaviors of SnO2 microsphere films modulated by top electrodes

Current Applied Physics 20 (2020) 431–437

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Rectifying resistance switching behaviors of SnO2 microsphere films modulated by top electrodes

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Rongchun Yuan, Weiwei Xia, Mengxue Xu, Zhilei Miao, Shudong Wu, Xiuyun Zhang, Junhui He∗∗, Qiang Wang∗ College of Physical Science and Technology, Yangzhou University, Yangzhou, 225002, China

A R T I C LE I N FO

A B S T R A C T

Keywords: Rectifying resistance switching SnO2 microspheres film Selfrectifying Metal electrodes

Based on the bipolar resistive switching (RS) characteristics of SnO2 films, we have fabricated a new prototypical device with sandwiched structure of Metal/SnO2/fluorine-doped tin oxide (FTO). The SnO2 microspheres film was grown on FTO glass by template-free hydrothermal synthesis, which was evaporated with various commonly used electrodes such as aluminium (Al), silver (Ag), and gold (Au), respectively. Typical self-rectifying resistance switching behaviors were observed for the RS devices with Al and Au electrodes. However, no obvious rectifying resistance switching behavior was observed for the RS device with Ag electrode. Above results were interpreted by considering the different interface barriers between SnO2 and top metal electrodes. Our current studies pave the ways for modulating the self-rectifying resistance switching properties of resistive memory devices by choosing suitable metal electrodes.

1. Introduction The technologies for fabrication of traditional memory devices are known to reach the physical limit. Resistive random access memories (RRAMs) are considered to be one of the promising substitutes for next generation nonvolatile memory [1], for they showing such outstanding properties as long retention time [2], high switching speed [3,4], small size [5], low power consumption [6], high density integration, simple structure, high-speed operation [7], nondestructive readout [8], and so on. Nowadays, many materials such as transition metal oxides, perovskite oxides, organic compounds, superconducting materials, etc. have been found to show resistive switching (RS) characteristics [9–13]. Due to the simple preparation process and excellent switching performance, the RS behaviors of metal oxides are drawing special attentions [14–18]. Resistive switching devices are considered to be very promising candidates for the applications in high-density memory array, analogues for biological synapses, integrated neural networks and so on [19,20]. As we know, RRAM is expected to be applied as the high-density memory array, but sneak-path leakage current among neighboring units is a key problem for common RS memory array applications, because leakage current can cause data-misreading for memory devices. As

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suggested [21–23], adding an extra selector in each RRAM unit can solve this problem. However, this method can also lead to other problems, such as higher process complexity, higher manufacturing costs, and larger package sizes. Many other solutions have been suggested to suppress the sneak-path leakage current in RS memory arrays by using self-rectifying RRAMs, such as non-uniform dielectric [22], asymmetric structures [24] and electrode-semiconductor interface barrier [25], etc. However, commercialization of these non-uniform dielectric and asymmetric structures is difficult in terms of manufacturing complexity and cost [26], and RS devices based on electrode-semiconductor interface are convenient and compatible to current manufacturing process. SnO2 is a wide bandgap metal-oxide semiconductor which has been widely used in many fields of oxide electronics, with many advantages such as excellent electrical and optical properties [27,28]. The intrinsic n-type semi-conductivity with inherent oxygen vacancies enable SnO2 to be used for memristive applications [29–31]. As we should speculate, SnO2 can be readily employed to form metal-semiconductor interface to obtain self-rectifying RS devices, so we have fabricated the resistive memory device by vertical stacking of SnO2 microspheres thin film. More defects can be introduced in the contact regions of adjacent SnO2 microspheres, which can enhance the carrier density and lower migration barrier [32]. And the contacts can induce a local electric field

Corresponding author. Corresponding author. E-mail addresses: [email protected] (J. He), [email protected] (Q. Wang).

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https://doi.org/10.1016/j.cap.2020.01.005 Received 28 September 2019; Received in revised form 4 January 2020; Accepted 6 January 2020 Available online 07 January 2020 1567-1739/ © 2020 Korean Physical Society. Published by Elsevier B.V. All rights reserved.

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Fig. 1(a), as well as the schematic diagram of the I–V measurement experimental device. The I–V measurement was performed at the base vacuum of 10−3 Pa in the SEM chamber. Two needle-like tungsten (W) probes were fabricated by the wet chemical etching methods of Miao et al. [37]. Two W probes were respectively loaded on the upper surface of the FTO and Metal electrode for I–V measurement, and the loading positions can be modulated by a computer-controlled precision motor for further testing.

enhancement at the micro-sized interfaces, and facilitate the migration of oxygen vacancies [33], which may be advantageous to RS properties. As contrast, B. Sun et al. had fabricated RS devices with Ag and Al electrodes [34,35], and the main conduction mechanisms are explained by the migration of the electrodes metal ions. The RS mechanisms for devices based on electrode-semiconductor interface are intrinsically complex, and there is currently no consensus. In the present work, we investigated the resistance change between the HRS and LRS by the migration of oxygen vacancies in the as-prepared Metal/SnO2/FTO thin film. This resistive switching device based on SnO2 microspheres film showed such excellent characteristics as simple fabrication, low manufacturing cost, and high ON/OFF ratio. Most strikingly, the memory device realizes the change of RS characteristics between self-rectifying and non-rectifying by selecting different metals (Au, Al, and Ag) as electrodes.

3. Results and discussion The morphology images of the SnO2 microspheres are shown in Fig. 1. Fig. 1(b) is the image of as-prepared SnO2 microspheres, which were tightly self-connected in a one-by-one manner, and shows a single SnO2 microsphere with a diameter of about 3 μm. The cross-sectional image of microspheres loaded onto FTO is shown in Fig. 1(c), and the films thickness is about 20 μm. Fig. 1(d) shows the high resolution transmission electron microscopy (HRTEM) image of the SnO2 microspheres. No other particles were found except SnO2, the lattice spacing between two planes is 0.236 nm, and it corresponds to (200) crystal planes of SnO2. As shown in Fig. 2(a), the X-ray diffraction (XRD) patterns were well matched with SnO2 (JPCDS card No. 41–1445), which reveals the overall crystalline structure and purity of the SnO2 microspheres. The XRD result also indicates that the SnO2 film are of polycrystalline structure. Fig. 2(b) shows the high-resolution XPS spectrum of SnO2 binding states. As shown in Fig. 2(c), the XPS spectrum of Sn 3d state reveals that peaks at 486.3eV and 494.7eV correspond to the Sn 3d5/2 and Sn 3d3/2 binding states respectively, which is similar to the standard results of Sn 3d binding states [37]. The resistance switching characteristics are intimately related to the content of oxygen vacancies. Therefore, the XPS spectrum of O 1s binding states are shown in Fig. 2(d), two peaks of the binding energy centered at 530.2eV and 531.5eV are assigned to lattice O2− species bound to Sn4+ and oxygen vacancies, respectively [38]. As we can see, there are a lot of oxygen vacancies defects, and they mainly locate at the interface regions where the SnO2 microspheres contact. SnO2 microspheres film with thickness about 20 μm was grown on FTO glass substrate by template-free hydrothermal synthesis, and then 100 nm top metal electrodes are plated with a heat evaporator. Two W

2. Fabrication and experimental methods Sodium hydroxide (NaOH), disodium tin trioxide (Na2SnO3·4H2O), urea (CH4N2O), absolute ethanol and acetone were purchased for the experiment, which used as received without any further purification. Before the growth of SnO2 film, the FTO substrates must be pretreated by ultrasonic cleaning with the mixed solution of absolute ethanol, acetone, and deionized water, and then activated by plasma. The SnO2 microspheres thin film grown on FTO wafer is fabricated by template-free hydrothermal synthesis, following the experimental process of Xia et al. [36]. Firstly, 0.034 M Na2SnO3·4H2O, 0.24 M NaOH and 0.13 M CH4N2O were dissolved in the mixed solution (40 mL of anhydrous ethanol and 160 mL of deionized water), and continuously stirred for 1 h to ensure complete mixing. The as-pretreated FTO substrate was placed at the bottom of a 50 mL sealed autoclave with Teflon liner, and the admixture was then poured into the sealed autoclave. The sealed heat reactor was then maintained at 160 °C for 3 h, and it was naturally cooled down to room temperature. The film was further washed with deionized water and anhydrous ethanol. Finally, the film was dried in a drying oven at 80 °C. The white SnO2 film can be observed growing on the substrates. The as-prepared contrast samples are respectively plated with Au and Al metal top electrodes (1.8 mm × 1 mm) by a thermal evaporator, the schematic diagram of Metal/SnO2/FTO device is shown in

Fig. 1. (a) Schematic diagram of the Metal/SnO2/ FTO RRAM device, and the sketch illustration of the setup for the I–V measurements. (b) SEM image of SnO2 microspheres. The inset is enlarged SEM image of SnO2 microspheres. (c) Cross-sectional image of microspheres loaded onto FTO. The SAED pattern of as-prepared SnO2 microsphere. (d) HRTEM image of individual SnO2 microsphere.

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Fig. 2. (a) XRD patterns for SnO2 microspheres. (b) High-resolution XPS spectrum of SnO2 binding states. (c) and (d) High-resolution XPS spectrum of O 1s and Sn 3d binding states.

Fig. 3. (a) 10 cycles of switching behaviors of Al/SnO2/FTO. (b) 10 cycles of switching behaviors of Au/SnO2/FTO. (c) 10 cycles of switching behaviors of Ag/SnO2/ FTO. (d) Current-voltage characteristics of memory devices witch different metal electrodes.

breakdown. Voltage bias was swept in the sequential order of 0 V → −20 V → 0 V→ +20 V → 0 V during the I–V measurement for ten sweeping circles. The device initially keeps insulating at a lower voltage regime, then the resistance suddenly decreases and the device abruptly

probes were loaded on the upper surface of FTO and metal electrode for I–V measurement. Driving voltage was biased on the FTO electrode while the metal electrode was grounded. As we see in Fig. 3, We provide a 2.5 mA safe current to prevent equipment from unrecoverable 433

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mechanism in the positive voltage region. To further study the different effects of top electrodes for RS devices, the schematic diagrams of energy band structure after the conductive filament has been formed under different bias conditions are shown in Fig. 5. Fig. 5(a) plots the energy band diagram of the Au/ SnO2/FTO device under negative and positive bias conditions. As we know, the work functions of SnO2 and FTO are 4.9 eV and 4.59 eV, respectively [42,43]. χSnO2 = 4.1eV is the electron affinity of SnO2 [44], and the work function of Au is 5.4 eV. As a result, a high interfacial barrier of 1.3 eV forms at the Au/SnO2 interface. SnO2 and FTO have similar work functions that can be considered as Ohmic mechanism, so the solely existing Au/SnO2 interface barrier dominates the electrical transport and RS characteristics in this device [27]. Under the condition of negative bias in Fig. 5(a) (left), The electrons injected from the SnO2 layer can be easily transported to Au, which enhances the electron carrier injection and increases the current greatly. In contrast, under high positive bias in Fig. 5(a) (right), the transport of electrons from Au is suppressed due to the formation of higher Interfacial barrier of Au/ SnO2. Therefore, the low current was observed and the self-rectifying effect is demonstrated. Above analysis well interprets the asymmetric bipolar resistive switching properties of the Au/SnO2/FTO device. The work function of Ag is 4.39 eV [45], the interface barrier for SnO2/Ag is only as low as 0.29 eV, the electrons can be transported easily without any apparent obstacles under positive bias or reverse bias as shown in Fig. 5(b). Therefore, we can observe the approximately symmetric bipolar resistive switching behaviors in Fig. 3(c). Al has a lower work function (~4.3 eV) [46] in comparison with Au (~5.4 eV), and it is even lower than that of SnO2. As a result, no rectifying barrier is plausibly present at the Al/SnO2 interface, which is rather different to that of Au/SnO2/FTO device. As we know, Al2O3 can be formed at the Al/SnO2 interface region during the evaporation of Al electrode [47], and Al/Al2O3 interface also exists. The oxidization of Al can even bring about abundant interfacial states, which will further complicate the discussion. All in all, new rectifying barrier is also present at the Al/SnO2 interface due to the oxidization of Al electrode, the electron affinity of Al2O3 is 1.4 eV [48], the higher interface barrier (~2.9 eV) of Al/Al2O3 is formed. Fig. 5(c) shows the energy band structure after the conductive filament has been formed under different bias conditions of Al/Al2O3 interface. The single-side metal/semiconductor barrier in the sandwiched structures of Al/SnO2/FTO will also bring about common rectifying bipolar resistive switching properties as shown in Fig. 3(a). As schematically illustrated in Fig. 6, the formation and rupture of the conducting filaments are attributed to the migration of oxygen vacancies, which reveals the electrical switching mechanism of the Metal/SnO2/FTO device. Fig. 6(a) shows the formation path for oxygen vacancy filaments in memory device with Au electrode, the conductive filaments are formed through the surface of SnO2 microspheres. Oxygen vacancies migrate from the top regions of SnO2 near electrode to the bottom regions near FTO electrode. The migration of oxygen vacancies is caused by the pre-existing defects in the contact regions of adjacent SnO2 microspheres at a given voltage [32,33]. The formation of the complete oxygen vacancy paths at the SET voltage causes the transformation of the device from HRS to LRS instantly [33]. Fig. 6(b) shows the formation of conductive filament through the microspheres in the LRS at the negative bias. When the negative voltage sweeps to the positive bias, the current is suppressed by the Au/SnO2 interface barrier, which causes self-rectifying effect. As shown in Fig. 6(c), during the RESET process, the conductive filament dissolves simultaneously. As a result, the device switches back to the HRS. Fig. 6(d–f) show the formation path for conductive filaments in memory device with Ag electrode. Oxygen vacancies and Ag + cations migrate towards the FTO electrode and form conductive filaments connecting under negative bias, which increases the conductivity [49]. Therefore, when we applied a negative sweep voltage Less than −12 V, we can observe that the current in the HRS increases, almost as much as in the LRS, and the

switches from HRS to LRS when the external voltage increases to the VSET. When the external voltage drops back to the VRESET, the resistance switches back to HRS consequently. We observed the obvious RS rectifying behaviors for the Al/SnO2/FTO and Au/SnO2/FTO devices as seen from Fig. 3(a–b), and the resistance switch window in the positive voltage region is very small compared to the negative voltage region. Fig. 3(d) shows the current-voltage characteristics of Al/SnO2/FTO device, during initial voltage sweep from 0 to −20 V, an abrupt increase of current was observed at about −14.7 V indicating the change from HRS to LRS. While a reverse voltage was applied from 0 V to 20 V, a current drop was observed at about 4.4 V indicating the transition from LRS to HRS, which is due to the delamination of the Al2O3 layer and SnO2 layer [26]. The VSET and VRESET with Au electrode are about −11 V and 20 V respectively. For the ten sweeping circles, the Au/ SnO2/FTO device exhibits much better sweeping stability than that of Al/SnO2/FTO device. The Ag/SnO2/FTO device shows the VSET and VRESET about −15.7 V and 17 V respectively as shown in Fig. 3(d). Comparing with the rectifying RS plots for Al/SnO2/FTO and Au/SnO2/ FTO devices, the Ag/SnO2/FTO device exhibited the symmetrically non-rectifying RS behavior as shown in Fig. 3(c), this is caused by different interface barriers, which will be discussed later. In order to better understand the RS behavior mechanisms of the devices with different top electrodes, the linear fitting curves of log(I)log(V) for the memory devices are shown in Fig. 4. As shown in Fig. 4(a) for Au/SnO2/FTO device, when the negative voltage is biased, the log(I)-log(V) curve shows a linearity slope of 1.19 to indicate the Ohmic conduction behavior at the LRS, which is typically due to the formation of conductive filaments in metal oxides [39]. However, at the HRS, the log(I)-log(V) characteristic is more complicated and could be divided into two regions. When the voltage ranges from 0 V to −8.8 V with a linearity slope of 1.14, it corresponds to the Ohmic mechanism. With an increased applied voltage, the current increases faster to follow the I ∝ V2 linear relationship, which indicates that the conduction mechanism is dominated by the space charge limited conduction (SCLC) [40]. When the positive voltage is biased as shown in Fig. 4(b), the slope is 4.2 for the LRS, corresponding to the SCLC mechanism [21]. For the HRS, the Schottky barrier at the Au/SnO2 interface limits the electrons injection from the Au to the SnO2 layer. As shown in the inset of Fig. 4(b), in low voltage region the curve is fitted to be a linear curve at ln(I) versus V1/2, which confirms that the carrier transport mechanism is dominated by Schottky emission [21,25]. As the voltage increases, the electrons in the defect states start to detrap continually because of the high electric field, and the inset of Fig. 4(b) shows that the ln(I/V) versus V1/2 plot corresponds to Poole-Frenkel emission [21,25,41]. The linear fitting results for Al/SnO2/FTO device are shown in Fig. 4(c) and (d). When the negative voltage is biased, the linearity slopes of 1.22 and 2.42 at the HRS are obtained, which respectively correspond to the Ohmic mechanism and the space charge limited conduction (SCLC) mechanism. And at the LRS it shows a linearity slope of 0.8 to indicate the Ohmic conduction behavior. At the positive bias as shown in Fig. 4(d), the slope is 1.7 for the LRS, corresponding to the SCLC mechanism. At the HRS, the line of low voltage region is fitted to be a linear curve at ln(I) versus V1/2, confirming that Schottky emission mechanism dominates the transport process. As the voltage continues to increase, the ln(I/V) versus V1/2 plot corresponds to Poole-Frenkel emission mechanism, as shown in the inset of Fig. 4(d). The conductive mechanism of Al/SnO2/FTO device are rather similar to that of Au/ SnO2/FTO device. As shown in Fig. 4(e) for Ag/SnO2/FTO device, when the negative voltage is biased, at a low voltage region, the slope of 1.16 indicates the Ohmic conduction behavior. When the voltage is increased to −13.5 V, the current increases faster to follow the I ∝ V2 linear relationship (slope of 2.47), corresponding to the mechanism of space charge limited conduction (SCLC). As the slope of LRS is close to 1, which conforms to the Ohmic conduction mechanism. Ag/SnO2/FTO device has symmetrical bipolar resistance characteristics and similar linear fitting results shown in Fig. 4(f), so it has the same conductive 434

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Fig. 4. Linear fitting results of log(I)-log(V) scale for different structures. (a) In negative and (b) positive voltage regions for the Au/SnO2/FTO structure, the insets in (b) are ln(I) versus V1/2 and ln(I/V) versus V1/2 plots for the Schottky emission and Poole-Frenkel emission. (c) In negative and (d) positive voltage regions for the Al/ SnO2/FTO structure, the insets in (d) are ln(I)–V1/2 and ln(I/V)–V1/2 plots for the Schottky emission and Poole-Frenkel emission. (e) In negative and (f) positive voltage regions for the Ag/SnO2/FTO structure.

from the Al2O3 layer and SnO2 layer to FTO electrode. As the voltage increases, the oxygen vacancies, as well as small amount of other ions, gradually form conductive filament as shown in Fig. 6(h), which transforms the device from HRS to LRS. At the positive bias, the current is suppressed by the Al/Al2O3 interface barrier, the self-rectifying effect is also observed. As shown in Fig. 6(i), under the subsequent RESET process, the conductive filament dissolves, the device switches back to the HRS. Since Al electrode has additional bonding with oxygen ions,

operating voltage window is greatly reduced shown in Fig. 3(c). The formation of the complete conductive filament paths at the SET voltage causes the transformation of the device from HRS to LRS instantly. As shown in Fig. 6(f), during the RESET process under the subsequent reversed bias, the conductive filament dissolves simultaneously and returns to initial state. Fig. 6(g–i) show the conductive filament formed in the LRS and ruptured conductive filament in the HRS in Al/Al2O3/ SnO2/FTO device. At the negative bias, the oxygen vacancies migrate 435

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4. Conclusions In summary, we have fabricated a new prototypical device with sandwiched structure of Metal/SnO2/FTO RS devices, and it demonstrated that the self-rectifying resistance switching properties can be modulated by choosing suitable metal electrodes. The migration of oxygen vacancies plays an important role in the RS transformation process. Because of the different interface barriers, the common rectifying RS properties of SnO2 can be realized by using Al and Au top electrodes, and Ag electrode memory device exhibited typical symmetrically non-rectifying resistive switching behavior. In this work, we have developed a memory device with simple structure, low manufacturing cost and good resistance characteristics, which opens up a broad prospect for SnO2-based RRAM devices, and provides the possibility to select suitable common electrodes for target storage applications. However, it is worth noting that the devices working at high operating voltage usually show excellent breakdown performance and large resistance switching ratio, but bring about the high energy consumption. The current work is not complete enough, and further improvement of technologies and methods is needed to meet the requirements of high integration. Declaration of competing interest

Fig. 5. Schematic diagrams of energy band structure after the conductive filament has been formed under different bias conditions. (a) Au/SnO2 interface, (b) Ag/SnO2 interface and (c) Al/Al2O3 interface.

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements Rongchun Yuan and Weiwei Xia contribute equally in this research, and they are Common First Author. We are thankful to the financial support of National Natural Science Foundation of China (NSFC) with the granted number of 11274082. References [1] S. Srivastava, P. Dey, S. Asapu, T. Maiti, Role of GO and r-GO in resistance switching behavior of bilayer TiO2 based RRAM, Nanotechnology 29 (2018) 505702. [2] J.W. Seo, J.W. Park, K.S. Lim, J.H. Yang, S. Kang, Transparent resistive random access memory and its characteristics for nonvolatile resistive switching, Appl. Phys. Lett. 93 (2008) 223505. [3] S.X. Chen, S.P. Chang, W.K. Hsieh, S.J. Chang, C.C. Lina, Highly stable ITO/ Zn2TiO4/Pt resistive random access memory and its application in two-bit-per-cell, RSC Adv. 8 (2018) 17622. [4] K.H. Chen, K.C. Chang, T.C. Chang, T.M. Tsai, K.H. Liao, Y.E. Syu, S.M. Sze, Hopping conduction properties of the Sn:SiOX thin-film resistance random access memory devices induced by rapid temperature annealing procedure, Appl. Phys. Mater. Sci. Process 119 (2015) 1609–1613. [5] R. Waser, M. Aono, Nanoionics-based resistive switching memories, Nat. Mater. 6 (2007) 833–840. [6] B. Sun, X.P. Li, D.D. Liang, P. Chen, Effect of visible-light illumination on resistive switching characteristics in Ag/Ce2W3O12/FTO devices, Chem. Phys. Lett. 643 (2016) 66–70. [7] F. Pan, S. Gao, C. Chen, C. Song, F. Zeng, Recent progress in resistive random access memories: materials, switching mechanisms, and performance, Mater. Sci. Eng. R Rep. 83 (2014) 1–59. [8] A. Beck, J.G. Bednorz, C. Gerber, C.L. Rosse, D. Widmer, Reproducible switching effect in thin oxide films for memory applications, Appl, Phys. Lett. 77 (2000) 139–141. [9] M. Truchly, T. Plecenik, E. Zhitlukhina, M. Belogovskii, M. Dvoranova, P. Pkus, A. Plecenik, Inverse polarity of the resistive switching effect and strong inhomogeneity in nanoscale YBCO-metal contacts, J. Appl. Phys. 120 (2016) 185302. [10] M. Ambrico, A. Cardone, T. Ligonzo, V. Augelli, P.F. Ambrico, S. Cicco, Hysteresistype current-voltage characteristics in Au/eumelanin/ITO/glass structure: towards melanin based memory devices, Org. Electron. 11 (2010) 1809–1814. [11] H. Kohlstedt, A. Petraru, K. Szot, A. Rüdiger, P. Meuffels, H. Haselier, R. Waser, V. Nagarajan, Method to distinguish ferroelectric from nonferroelectric origin in case of resistive switching in ferroelectric capacitors, Appl. Phys. Lett. 92 (2008) 062907. [12] C. Schindler, G. Staikov, R. Waser, Electrode kinetics of Cu-SiO2-based resistive switching cells: overcoming the voltage-time dilemma of electrochemical metallization memories, Appl. Phys. Lett. 94 (2009) 072109.

Fig. 6. Schematic illustration of the Metal/SnO2/FTO device in the LRS and HRS. The orange regions represent the oxygen vacancy path. (a) Oxygen vacancy path, (b) Conductive filament formed in the LRS, (c) Ruptured conductive filament in the HRS in Au/SnO2/FTO device. (d) Oxygen vacancy path, (e) Conductive filament formed in the LRS, (f) Ruptured conductive filament in the HRS in Ag/SnO2/FTO device. (g) Oxygen vacancy path, (h) Conductive filament formed in the LRS, (i) Ruptured conductive filament in the HRS in Al/ Al2O3/SnO2/FTO device. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

the Al2O3 layer is formed, which results in a higher SET voltage and more complex I–V characteristics. As discussed above, Au/SnO2 interfacial electric field activates the migration of oxygen vacancies in SnO2. For the case of Al/Al2O3/SnO2 cascaded interface, the electric field is shared and decreased at the local Al2O3/SnO2 interfaces, which will weaken the migration of oxygen vacancies in SnO2. It resultantly leads to the weakened RS properties of Al/SnO2/FTO device in comparison with the counterpart with Au electrode.

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